Polyethylene terephthalate (PET, PETE, or recycle symbol “1”) is one of the most frequently used consumer plastics. Everyday products made from PET include: water bottles, soda bottles, food packaging, clothing, carpeting and filler for furniture. The United States alone consumes more than two million tons of PET plastic annually, with demand increasing each year. This demand puts a growing strain on local landfills and an ever-increasing burden on the environment, as PET is produced from petrochemicals.
To address increasing PET demand while reducing the environmental impact of PET production, efforts to recycle PET have been employed. Although the most common PET recycling techniques are in-plant and mechanical methods, chemical recycling is considered to be the only sustainable option for the recycling of PET.
Chemical recycling is the act of breaking the polymer down to either its original starting materials or its reactive intermediates. Breaking the PET down to small molecules allows recyclers the ability to remove contaminants (dyes, adhesives, bottle caps) using practices from industrial chemistry (precipitation, decantation, solvation etc.). By breaking down the PET into monomers and oligomers that are readily purified, the finished polymer is suitable for food-contact without any further treatment.
Glycolysis of PET is one of the most widely studied chemical recycling processes. PET depolymerization by glycolysis involves heating PET and ethylene glycol in the presence of a catalyst. This results in the degradation of PET to a reactive monomer—bis(2-hydroxyethyl) terephthalate monomer (BHET)—which then can be purified and re-polymerized to form new, virgin PET.
Because chemical PET recycling via glycolysis requires exotic catalysts and significant amounts of energy (heat), current research has been focused on discovering new catalysts, optimizing reaction conditions, and implementing new processes for achieving more efficient degradation of PET. One area of particular interest is the utilization of microwave irradiation to degrade PET. Microwave heating leads to extremely short reaction times with much higher energy-to-heat conversion when compared to conventional heating means. However, even with the use of microwave irradiation, PET recycling via glycolysis is still cost prohibitive due to the energy requirements.